The present invention relates, in a projection aligner wherein a mask and a wafer are held proximate to one another and wherein a circuit pattern depicted on the mask is transferred onto the wafer, to a method of detecting the respective positions of the mask and the wafer for the relative positioning between the mask and the wafer.
Heretofore, in the relative positioning between a mask and a wafer, positioning marks have been detected by a microscope as described in, for example, Japanese Patent Application Laid-open No. 57-4291. With such a system, however, the objective of the microscope is upright over the planes of the mask and the wafer, so that an optical system for the positioning mark detection and projection light, for example, an X-ray interfere in case of transferring the circuit pattern of the mask onto the wafer through projection. For this reason, the objective of the microscope needs to be withdrawn from an optical system for the projection during the transfer of the circuit pattern. The withdrawal requires a long time, and has led to the problem that the throughput lowers.
For example, regarding present-day LSI's, as the stripe width thereof is becoming 1 .mu.m or less, an X-ray aligner which is a typical proximity aligner is directed to a stripe width of or below 0.8 .mu.m. Therefore, an alignment precision within 0.3 .mu.m has been required.
As methods of realizing alignment detection, there are a method wherein alignment patterns are magnified and detected, a method wherein a laser is scanned, a method wherein a position is magnified by the use of a diffraction grating, etc.
With the method which magnifies and detects the alignment patterns, the alignment patterns of a mask and a wafer are magnified and detected by an objective, and they are focused on an imaging device so as to perform alignment through signal processing. Therefore, in order to attain the aforementioned alignment precision within 0.3 .mu.m, the precision of the alignment detection itself needs to be a high precision within 0.1 .mu.m because the mechanical errors of the aligner and the dimensional errors of the mask and wafer themselves are involved besides the alignment detection precision.
In this regard, there has hitherto been invented, for example, a measure wherein as described in the official gazette of Japanese Patent Application Publication No. 57-42971, an objective of small focal depth is used thereby to enlarge the aperture of the lens and to take a larger quantity of light, making it possible to image patterns of inferior contrast.
In the case of using the objective of small focal depth as described above, however, when it is intended to obtain a pattern image of high precision, the magnifying power of the lens becomes 40-60 magnifications and the NA (Numerical Aperture) becomes 0.5 or more, and also the aperture enlarges and the working distance is small. Accordingly, when the alignment patterns lie in a projection region, the objective might interfere with an X-ray for projection.
Therefore, an objective set needs to be withdrawn at each projecting operation, resulting in the problems that a high throughput is hampered and that the alignment patterns cannot be detected during the projection.
In still another prior-art example illustrative of a method and apparatus for positioning a mask and a wafer, Fresnel zone marks provided on the mask and the wafer are obliquely illuminated from outside a projection region toward the projection region, and hence, coherent light diffracted to Fresnel zones is focused in the projection region. Therefore, an objective for detecting the focused position is inevitably arranged in the projection region. Accordingly, after the focus in the Fresnel zones has been detected by the objective and the wafer and mask have been aligned, the objective must be withdrawn. Since the projection in the prior art employs a system of exposing the whole surface of the wafer to projection light collectively, one alignment operation suffices for one wafer, so that a somewhat long time required has not been a problem. Pertinent to the positioning method of this type is, for example, Japanese Patent Application Laid-open No. 56-157033.
With a step-and-repeat system aimed at the transfer of ultrafine patterns of 1 .mu.m or less, however, ten odd times of alignment and projection operations are repeated per wafer, and hence, it is an important theme to shorten the alignment period of time.
Another problem is that, since the objective is moved after the alignment between the mask and the wafer, the vibrations thereof degrade the alignment precision of the mask and the wafer.